Recently, video signals have being digitized, and digital broadcast has been started around the world in broadcasting media of satellite, CATV, and terrestrial waves. As the transmission system, a system suited to features of each transmission line is selected. For example, phase shift keying such as 4PSK or 8PSK is employed in satellite broadcast, or quadrature amplitude modulation such as 64QAM or 256QAM is employed in CATV.
Demodulation systems of such digital-modulated signals are proposed in various publications, and, for example, by referring to “A study on QPSK demodulation system” by Taga, Ishikawa, and Komatsu (ITEJ Technical Report, vol. 15, No. 46, CE' 91-42 (August 1991), a conventional carrier recovery apparatus is explained.
FIG. 30 is a block diagram showing a configuration of carrier recovery apparatus in a prior art. In FIG. 30, the carrier recovery apparatus in the prior art comprises a modulated signal input terminal 5010, a complex multiplier 5011, an arc tangent arithmetic unit 5030 for calculating an arc tangent, a loop filter 5013, a numerical control oscillator 5014, and a demodulated signal output terminal 5015.
In FIG. 30, a signal line indicated by thick line and “/2” indicates a signal line of signal expressed by complex.
The operation of the carrier recovery apparatus in the prior art is briefly described below.
In FIG. 30, a received digital-modulated signal is once demodulated in quadrature in a front stage, and put into the modulated signal input terminal 5010. At the time of quadrature detection in the front stage, however, the carrier for quadrature detection is not always in accurate frequency and accurate phase. Therefore, the signal entered in the modulated signal input terminal 5010 involves a residual discrepancy of frequency and phase. The signal once fed into the modulated signal input terminal 5010 is further put into one input terminal of the complex multiplier 5011. The numerical control oscillator 5014 outputs a complex oscillation signal composed of two mutually orthogonal oscillation signals, and it is put into other input terminal of the complex multiplier 5011.
The complex multiplier 5011 multiplies the output of the numerical control oscillator 5014 and the signal entered in the modulated signal input terminal 5010, and removes the frequency and phase discrepancy of the signal entered in the modulated signal input terminal 5010, and outputs a demodulated signal through the demodulated signal output terminal 5015.
On the other hand, demodulated signals Si and Sq as outputs from the complex multiplier 5011 are put into the arc tangent arithmetic unit (Tan−1) 5030. The arc tangent arithmetic unit (Tan−1) 5030 calculates the arc tangent on the basis of the values of Si and Sq, and detects the phase error between the carrier signal of the digital-modulated signal supplied in the modulated signal input terminal 5010 and the output signal of the numerical control oscillator 5014. The output of the arc tangent arithmetic unit 5030 is put into the loop filter 5013, and the high frequency component of the phase error is eliminated. Thus, the output of the loop filter 5013 is put into the numerical control oscillator 5014 as the control signal to the numerical control oscillator 5014. The output signal of the numerical control oscillator 5014 controlled by the output signal of the loop filter 5013 is supplied into the complex multiplier 5011.
In the explanation above, as shown in formula (1) and formula (2), the output signal of the numerical control oscillator 5014 is a signal in conjugate relation with the carrier signal of the signal entered in the modulated signal input terminal 5010 (that is, free from frequency discrepancy and phase discrepancy). Therefore, as far as the relation for formula (1) and formula (2) is satisfied, the arc tangent arithmetic unit (Tan−1) 5030 detects zero phase error. When there is a phase difference between formula (1) and formula (2), the arc tangent arithmetic unit (Tan−1) 5030 outputs a signal corresponding to the phase error.
Since the negative feedback control loop is composed by the phase control loop thus composed, the carrier synchronized in phase with the received digital-modulated signal is reproduced in the numerical control oscillator 5014. This reproduced carrier is in conjugate relation with the carrier signal of the signal entered in the modulated signal input terminal 5010 (that is, free from frequency discrepancy and phase discrepancy), and is free from frequency error and phase error, so that a correct demodulated signal may be obtained.
As mentioned above, the phase error detection in a conventional carrier recovery circuit is performed by arc tangent calculation of the output of the complex multiplier 5011 by the arc tangent arithmetic unit 5030. This operation is further described below by referring to FIG. 31.
FIG. 31 is an output signal space diagram for explaining the operation of conventional phase error detection. Herein, the digital-modulated signal to be received is assumed to be 4PSK, and for the ease of explanation, only the first quadrant is explained. Assume there is a phase error of Δθ between the digital-modulated signal entered in the modulated signal input terminal 5010 and the output signal of the numerical control oscillator 5014. The demodulated signal which is the output of the complex multiplier 5011 is indicated by mark “◯” 5041. If an accurate carrier is reproduced, there is no phase error Δθ, an hence the demodulated signal which is the output of the complex multiplier 5011 is indicated by mark “mark “●” 5042 which is the intrinsic phase of the symbol of 4PSK. The phase indicated by mark “mark “●” 5042 is (π/4+n·π/2) [radian] (n=0, 1, 2, 3). However, since the presence of phase error Δθ is assumed, the output signal from the complex multiplier 5011 is present at the position of phase φ (φ=π/4+Δθ).
This phase error Δθ was calculated by obtaining the phase φ by calculating the arc tangent on the basis of outputs Si and Sq of the complex multiplier 5011, and obtaining the difference of the phase φ of this reception symbol and the symbol phase (π/4) of the intrinsic 4PSK.
Incidentally, the calculation of arc tangent for obtaining the phase φ of the reception symbol is generally performed by storing the value of Tan−1 (Sq/Si) preliminarily calculated in Si and Sq generally in a storage device such as ROM, and reading out by using Si and Sq as the address. Or, by calculating the rotation of two-dimensional vector on the basis of Si and Sq, the angle of the portion of rotation is obtained, which is known as Cordic algorithm.
However, the calculating method of arc tangent by using the ROM requires an enormous ROM capacity, and the circuit scale increases. The Cordic method requires many steps to obtain the phase of high precision, and the frequency capture range becomes narrow due to increase in the delay in loop of the carrier recovery circuit.